dm: only run the queue on completion if congested or no requests pending

On really fast storage it can be beneficial to delay running the
request_queue to allow the elevator more opportunity to merge requests.

Otherwise, it has been observed that requests are being sent to
q->request_fn much quicker than is ideal on IOPS-bound backends.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>

diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index 43e0d1a..7924c00 100644 (file)
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -1024,10 +1024,13 @@ static void end_clone_bio(struct bio *clone, int error)
  */
 static void rq_completed(struct mapped_device *md, int rw, bool run_queue)
 {
+       int nr_requests_pending;
+
        atomic_dec(&md->pending[rw]);
 
        /* nudge anyone waiting on suspend queue */
-       if (!md_in_flight(md))
+       nr_requests_pending = md_in_flight(md);
+       if (!nr_requests_pending)
                wake_up(&md->wait);
 
        /*
@@ -1036,8 +1039,11 @@ static void rq_completed(struct mapped_device *md, int rw, bool run_queue)
         * back into ->request_fn() could deadlock attempting to grab the
         * queue lock again.
         */
-       if (run_queue)
-               blk_run_queue_async(md->queue);
+       if (run_queue) {
+               if (!nr_requests_pending ||
+                   (nr_requests_pending >= md->queue->nr_congestion_on))
+                       blk_run_queue_async(md->queue);
+       }
 
        /*
         * dm_put() must be at the end of this function. See the comment above